The present invention relates to a magnetic carrier for a developer, which is used as a constituent of a developer for developing electrostatic latent images in electrophotographic recording apparatus such as printers, facsimiles, copying machines, etc.
In conventional electrophotographic recording apparatus, it is known that a visual toner image is produced by the successive steps of (1) forming an electrostatic latent image corresponding to original image or information data, for instance, on a photosensitive surface of a cylindrical image-bearing member, (2) magnetically attracting a magnetic developer comprising a magnetic carrier and a magnetic or non-magnetic toner on a rotatable developing roll equipped with an inner permanent magnet and disposed opposing the image-bearing member, (3) delivering the magnetic developer, while forming a magnetic brush, to a developing zone where the electrostatic latent image on the image-bearing member is slidingly brushed with the magnetic brush. The developed image is then transferred to a recording sheet and fixed thereon by heating.
In the conventional magnetic developer, a magnetic carrier of spherical or non-spherial granular shape is usually used in view of improving flowability of the developer. However, a small specific surface area of the known spherical or non-spherial granular magnetic carrier inevitably leads to a small contact area of the carrier and the toner. Therefore, the toner cannot be sufficiently charged by the carrier to obtain a small amount of triboelectric charge, thereby failing to obtain a clear toner image because an insufficient amount of triboelectric charge of the toner usually causes printing defect such as fogging, etc.
Further, proposals have been made to form a fine toner image by reducing a size of the magnetic carrier. By using the magnetic carrier having a reduced size, a toner image with a high resolution and a high quality can be obtained due to the formation of a thin developer layer on a sleeve of a developing means. However, since the magnetic carrier with a reduced size fails to be well magnetically retained on the developing means, the magnetic carrier is likely to scatter, thereby causing problems such as the contamination of the developing means and nearby elements, deterioration in a quality of the toner image, etc.
A magnetic carrier of flat shape having a large particle size exceeding 100 .mu.m has been also proposed. However, such a magnetic carrier has problems of damaging a photosensitive surface or causing streaks in printed images because of vigorous moving of the magnetic carrier accompanied by the rotation of a developing roll.